1. Field of the Invention
The present invention relates to an optical recording apparatus and particularly to an optical recording apparatus capable of writing new information while erasing already written information.
2. Description of the Prior Art
There is known an optical recording system in which a laser beam is focused on a vertical magnetic film formed on a substrate and an external magnetic field is applied to the vertical magnetic film heated by the beam to a temperature higher than the Curie point to reverse a direction of magnetization, thereby to record information. In this system, the information is reproduced in a manner in which a laser beam having lower power than that for recording is focused on the vertical magnetic film to detect rotation of a polarized face of a reflected beam of the laser beam caused by a magnetic optical effect such as a Kerr effect or a Faraday effect. Such an optical recording and reproducing system is advantageous in that information can be rewritten, compared with a system in which information is recorded by perforating a metal film. For this reason, such an optical recording and reproducing system has attracted special interest as an excellent recording and reproducing system.
FIG. 1 is a view showing an example of a conventional optical recording and reproducing apparatus using the above described optical recording and reproducing system. This example of FIG. 1 is disclosed in Japanese Patent Laying-Open No. 117106/1982. In this conventional apparatus, an information signal and an address signal are recorded on the same track of an optical magnetic disk D. The address signal has been recorded for each rotation of the disk D for example, namely, on the same line in a radial direction (for example in the form of concavities and convexities) and can not be erased by an erasing beam. This conventional apparatus is capable of recording new information by a recording beam applied to a track while it is erasing already recorded information by an erasing beam applied to a track preceding that track. More specifically, the erasing beam and the recording beam are applied to tracks adjacent to each other or tracks spaced by an amount corresponding to two tracks or more. The erasing beam detects an address to change a bias magnetic field alternately for each track. Thus, new information can be recorded by the recording beam while already recorded information is being erased by the erasing beam.
More specifically stated, a beam from a laser diode 1 for generating an erasing beam is caused to be a parallel beam through a collimating lens 2. The parallel beam passes through a beam splitter 3 and a half mirror 4 and then it passes through an objective lens 5 so that it is focused on the disk D having a surface on which a vertical magnetic film is formed). Part of a reflected beam from the disk D is reflected on the half mirror 4 and then passes through a lens 6 and a beam splitter 7 so that it is applied to a photo sensor 8 having two divided regions. The photo sensor 8 has the two regions for receiving light, which are divided with respect to a face perpendicular to the surface of the drawing, including an optical axis of a reflected beam from the disk D applied to the photo sensor 8. Since brightness patterns of light applied to the two light receiving regions are changed dependent on a positional relation between the erasing beam and track grooves when a depth of a track groove of the disk D is a value other than an integral multiple of 1/4 of a wavelength of the erasing beam, the photo sensor 8 is capable of obtaining a tracking error signal by obtaining a difference of output signals of the two light receiving regions by means of a differential amplifier 9. In addition, by obtaining a sum of the output signals of the two light receiving regions by means of a summing amplifier 10, an address signal can be obtained. Based on this address signal, a current direction of a magnetic field generating coil 11 is reversed for each track so that a direction of magnetic field to be applied is reversed for each track to erase recorded information.
On the other hand, a beam from a laser diode 12 for generating a recording beam is caused to be a parallel beam through a collimating lens 13 and then it is linearly polarized through a polarizer 14 such as a Glan-Thompson prism and is reflected on the beam splitter 3 so as to be focused on a track of the disk D (a track located one track backward from that on which the erasing beam is focused) by means of the objective lens 5. The laser diode 12 for recording is driven by applying an input signal according to information to be recorded, the polarity of the input signal being inverted for each track (based on the address signal from the summing amplifier 10). As a result, the direction of magnetization of the vertical magnetic film on the disk D is reversed according to information to be recorded (the direction of magnetization being changed for each track) so that the information is recorded in the form of reversing of the direction of magnetization).
The recorded information is reproduced in the following manner. First, the laser diode 12 for recording is continuously oscillated with a small output value. A beam from the laser diode 12 is caused to be a parallel beam through the collimating lens 13. The parallel beam passes through the polarizer 14 and is focused on the disk D. A reflected beam from the disk D passes through the half mirror 4, the lens 6 and the beam splitter 7 and attains an analyser 15 such as a Glan-Thompson prism. As described above, the disk D is magnetized in a direction perpendicular to a recording face and the information is recorded according to the difference of the directions of magnetization. Consequently, the reflected beam of the linearly polarized laser beam applied to the disk D has a polarizing face rotating according to the direction of magnetization. The analyser 15 converts a difference of the rotating directions of the beam to a difference of intensity of the beam so that the beam obtained after the conversion is applied to a reproduced signal detecting photo sensor 16, where the beam is converted to an electric signal. Thus, a reproduced signal corresponding to the recorded information can be obtained from the photo sensor 16. The magnetic field generating coil 11 is not driven at the time of reproduction.
Since the laser diode 1 for erasing and the laser diode 12 for recording are provided separately in the above described conventional apparatus shown in FIG. 1, the apparatus involves disadvantages that it is complicated and has a large size and alignment of an optical system of the apparatus causes inconvenience in applying the erasing beam and the recording beam to prescribed tracks with a prescribed spacing or in bringing the beams into focused state on the disk. In addition, although the reflected beams of the erasing beam and the recording beam from the disk D attain the beam splitter 7 at the time of erasing and recording information, accuracy for applying the recording and reproducing beams on the disk with a prescribed spacing becomes less severe by adopting, in the beam splitter 7, a mirror capable of selecting wavelength. However, if the wavelengths of the two beams are close to each other, separation of the two beams by the beam splitter 7 (separation by the wavelengths of the beams) can not be effected satisfactorily. Accordingly, the reflected beam of the recording beam subjected to the modulation based on the recording information also attains the photo sensor 8 which should receive only the reflected beam of the erasing beam to detect an address signal, causing incorrect reading of the address signal.